1. Field of the Invention
The present invention relates to a device for the digital demodulation of a signal received in respect of a digital communication receiver comprising:
a stage for sampling the signal received with an oversampling factor of P samples per symbol;
a stage for chopping the oversampled signal into successive blocks of samples; and
means for correcting, for each block, the sampling instant, which means for correcting the sampling instant comprise:
a polyphase filter comprising a set of N distinct elementary digital filters; and
means of selection, for each block of an elementary digital filter.
2. Description of the Related Art
In installations for transmitting a signal, between a sending entity and a receiving entity, the digital signal comprising the data to be transmitted, represented by a baseband signal, is modulated into a signal suitable for transmission, in particular over the airwaves.
The receiving entity, or receiver, comprises a device for demodulating the signal received making it possible to reconstruct a baseband signal representative of the data transmitted. This demodulation implements a sampling of the modulated signal for its digitization.
The modulation of the signal and the demodulation of the signal are performed on the basis of a time base imposed, respectively, by the clock of the sending entity and by the clock of the receiving entity.
The clocks of the two entities are only rarely exactly synchronized in terms of frequency and phase, so that a time shift exists between the time bases supplied by the two clocks.
Now, for the digitization of the signal received, which is performed during demodulation, it is necessary for the sampling instants to be placed perfectly so as to utilize the performance of the receiving entity to the maximum. The sought-after position corresponds to the centre of the eye diagram which corresponds to minimizing the inter-symbol interference. However, even if the components involved in the analogue/digital conversion of the receiving entity are perfect, it is not possible to sample the signal at the right instance without a sophisticated feedback system. Such a system ensures correction of the sampling instant in respect of the signal received.
The known systems implement either analogue procedures, which are expensive in terms of components and room, or digital procedures which are expensive in terms of computation time.
The digital procedures customarily perform a double interpolation/decimation, the term “decimation” designating an undersampling. This double interpolation/decimation consumes a great deal of computational power, on the one hand in order to carry out the two functions themselves, and on the other hand, because the interpolation function increases the number of samples, and hence the number of computations to be performed pursuant thereto.
Moreover, the use of a polyphase filter in a receiving entity has been envisaged. Such a polyphase filter comprises a battery of several filters installed in parallel. Only one of the filters is used for the processing of a block of symbols.
In certain devices, the means implemented for the selection of the elementary filter of the polyphase filter perform continuous analysis of the incoming signal and directly determine the elementary filter to be used.
This solution is effective but is however complex to implement and can, in certain circumstances, be unstable.
The aim of the invention is to provide a simple device for the digital demodulation of a signal received ensuring correction of the sampling instant which is simple and allows stable operation.